Fix Select.attachedTo (backport #4458)

[mergify[bot]]

Previously it would always fail to find anything.

Working on the Mill build, I was lazy in setting up scalacOptions and applied -Xlint:infer-any to everything which caught this. Unfortuantely, Iterable.contains is not type safe but if you screw up the type, this lint will catch it inferring Any (or Object).

Contributor Checklist

• [ ] Did you add Scaladoc to every public function/method?
• [x] Did you add at least one test demonstrating the PR?
• [x] Did you delete any extraneous printlns/debugging code?
• [x] Did you specify the type of improvement?
• [x] Did you add appropriate documentation in docs/src?
• [x] Did you request a desired merge strategy?
• [ ] Did you add text to be included in the Release Notes for this change?

Type of Improvement

• Bugfix

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

• [ ] Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• [ ] Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• [ ] Did you review?
• [ ] Did you check whether all relevant Contributor checkboxes have been checked?
• [ ] Did you do one of the following when ready to merge:
  • [ ] Squash: You/ the contributor Enable auto-merge (squash), clean up the commit message, and label with Please Merge.
  • [ ] Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

---

This is an automatic backport of pull request #4458 done by [Mergify](https://mergify.com).